1. Field of the Invention
This invention relates generally to thermal printheads which are used in facsimile machines and various printers. More specifically, the present invention relates to a thermal printhead of the type which comprises a plurality of unit substrates arranged in series on a base board for providing an increased printing width.
2. Description of the Prior Art
As is well known, thermal printheads are widely used to print information on papers of various sizes. Thus, the length of the printhead (namely, the printing width) must be adjusted to suit the particular paper size to which the printhead is applied.
However, if the paper size is too large, it becomes difficult or impractical to increase the length of a single thermal printhead to suit the excessively large paper size such as JIS-A2 (JIS: Japanese Industrial Standards) or larger paper size. Further, it is technically disadvantageous to provide thermal heads of various sizes due to the necessity of redesigning upon every change in size.
On the other hand, for enabling a printing operation of a thermal printhead, a drive current is made to pass through a common electrode which is connected commonly to a multiplicity Of heating dots (divided portions of a linear heating resistor). Thus, if the printhead (namely, the common electrode) is rendered long for adaptation to a large paper size, a problem of voltage drop will arise with respect to those heating dots located far from the power supplying point or points of the common electrode due to the inherent resistivity of the common electrode, consequently resulting in a deterioration of the printing quality.
In view of the above problems, Japanese Patent Application Laid-open No. 4-52149 (Laid-open: Feb. 20, 1992) of the same inventor proposes a thermal printhead which overcomes both of the problems described above.
More specifically, the prior art thermal printhead disclosed in the above Japanese document comprises a plurality of unit substrates arranged in series on a common base board. The unit substrates carry their respective heating resistors aligned in a single straight line, and their respective common electrodes extending along the line of heating resistors. Further, the base board carries a passage widening conductor strip which is electrically connected to the common electrodes of the respective unit substrates and to a single power source.
The above-described thermal printhead is called "divisional-type thermal printhead" due to the use of plural unit substrates. In such a printhead, the overall printing width can be optionally adjusted by selecting the number of unit substrates without changing the length of each unit substrate itself. Further, the passage widening conductor strip formed on the base board reduces the voltage drop along the common electrodes of the respective unit substrates, thereby preventing a deterioration (unevenness) of the printing quality which would result from such a voltage drop.
However, the divisional-type thermal printhead described above is still unsatisfactory for the following reasons.
In the prior art divisional-type thermal printhead, the passage widening conductor strip is used commonly for the common electrodes of the respective unit substrates. Thus, the respective common electrodes of the unit substrates are held at a constant voltage level because there is substantially no voltage drop along the passage widening conductor strip.
However, it is difficult to equalize the resistivity characteristics with respect to the heating resistors of the respective unit substrates due to inevitable manufacturing variations. Particularly, the resistivity characteristics tend to vary more greatly between different lots than in a same lot. Further, the resistivity characteristics variation is more pronounced when the heating resistor is formed by a thin-film printing method than when it is formed by a thick-film printing method.
If a same voltage is applied to the heating resistors (namely, the common electrodes) of the respective unit substrates which have different resistivity characteristics, it is impossible to equalize the printing quality between the respective heating resistors. Thus, in the prior art printhead, it is only possible to equalize the printing quality within each heating resistor due to the function of the passage widening conductor strip, and such a function of the passage widening conductor strip gives rise to a new problem of printing quality inequality between the different unit substrates.
It is conceivable to select plural unit substrates of a uniform resistivity characteristics before incorporation into the printhead, thereby equalizing the printing quality along the entire length of the printhead. However, this solution is impractical in that the selection of unit substrates is troublesome and requires discarding of those unit substrates which are outside an acceptable limit, thus resulting in a cost increase.